1. Field of the Invention
The present invention relates to a surface coated cutting tool comprising a hard layer formed on the surface and having cutting characteristics such as resistance to fracture and wear.
2. Description of Related Art
Conventionally, a surface coated cutting tool having a hard layer formed on the surface of a substrate has been widely used. In many cases, a tool wherein a hard layer composed of a single layer or a multilayer of layers such as TiC layer, TiN layer, TiCN layer, Al2O3 layer and TiAlN layer is formed on the surface of a hard substrate such as cemented carbide, cermet and ceramics has been used as the cutting tool.
Cutting processes are carried out with such a cutting-tool largely in wet cutting, where operations are carried out throwing a cutting fluid mainly composed of water on a cut part in order to reduce temperature rise of a cutting edge during cutting processes.
Recently, as cutting processes become more highly efficient, high speed cutting processes are increasingly carried out. Even in wet cutting, a cut part such as a rake face and a cutting edge sometimes reaches very high temperature. Consequently, a conventional tool is tend to cause lowering wear resistance, film peeling, notch wear, poor accuracy in operations due to plastic deformation of a cutting edge and increased cutting resistance. These cause sudden tool damage such as fracture of a cutting edge and occurrence of abnormal wear, making it difficult for a cutting tool to have a longer life.
In particular, there have been such problems as the high-temperature state of a rake face and a cutting edge, progress of crater wear due to deterioration of a hard layer and film peeling.
On the other hand, a flank face does not come into as high-temperature state as a rake face and a cutting edge. However, in other cases than wet high-speed cutting processes, a cutting edge linked to a flank face reaches high temperature as well and therefore a work material is tend to adhere to a flank face. When a work material adheres to a flank face of a tool, there are problems such as poor operation accuracy and deteriorated surface roughness of a work material. In addition, when the adhering material is removed, damage to a cutting edge such as peeling of a hard layer and chipping is tend to occur. In particular, at the time of high-speed cutting processes for finished surface, adhesion of a work material to a flank face has been a large problem.
It is well known that in the above cutting tool, characteristics of a cutting edge have large influence on cutting characteristics. For example, Japanese Unexamined Patent Publication No. H8-11005 discloses that in a cutting edge of a surface coated cutting tool wherein a multilayer film including an oxide layer is formed, removal of part of a multilayer film including an oxide layer improves resistance to peeling and fracture of a hard layer.
Furthermore, Japanese Unexamined Patent Publication No. 2004-50385 discloses that by polishing the surface of a surface coated cutting tool having a hard layer formed and obtaining a polished surface smooth enough to gloss the whole tool, friction coefficient of a cutting edge and a work material during cutting is made small and heat generation in a cutting edge and adhesion of a work material are prevented.
However, a smoothed surface obtained by processing the surface including a cutting edge of a hard layer as mentioned in these publications reduces cutting resistance and improves tribology property, but does not sufficiently spread a cutting fluid throughout the surface of a tool. In particular, a cutting fluid cannot sufficiently spread through a rake face which reaches very high temperature during cutting.
This makes it impossible to sufficiently attain cooling effect by a cutting fluid and makes it difficult to quickly release the heat generated by cutting. There has been also the problem that even in wet cutting where heat is relatively hardly generated, in case of high-speed cutting processes, suddenly decreased wear resistance shortens a tool life.
Moreover, on a flank face which has large influence on accuracy of the finished surface of a work material, a smoothed surface obtained by processing the surface including a cutting edge of a hard layer is not enough alone. Namely, cutting resistance is reduced and tribology property is improved, but in case of high speed cutting, adhesion of a work material to a flank face cannot be inhibited, resulting in separation of an adhering work material, film peeling accompanying this and tool damage such as chipping of a cutting edge.